1. Field of the Invention
The present invention relates generally to gravity drop percussion drills, and more particularly to an improved multiple blow percussion drill assembly having specially configured internal components to increase its service life and to facilitate rapid field repairs.
2. Description of the Prior Art
Methods and apparatus for drilling holes in the earth as needed for the sinking of deep wells, and the like, have a long history of development. The need to drill these wells in rocky terrain gave rise to the early and well known technique of percussion drop drilling wherein a cable suspended from a drill rig is fitted with a cutting tool or bit at its lower end, all of which is alternately hoisted and abruptly dropped to effect the desired crushing/drilling action. Generally, powered equipment is used to hoist the heavy components involved. Energy converted from the gravity drop affects the cutting action, and drill bits of hardened steel which may include carbide tips are employed. This basic approach has been refined over the years to include improved percussion drilling devices which deliver multiple blows based on stored energy elements within the drill bit assembly. Whereas the early percussion drill techniques used a fairly simple and solid drill stem, the multiple blow devices called for hollow drill stems having internal hammer/anvil/spring components. Significantly improved penetrations were achieved as the energy of the internal hammer striking the internal anvil added to the initial drill bit impact just at the time when the bit and rock formation are under compression. Descriptions of typical prior art drills of this multi-blow type are found in U.S. Pat. Nos. 3,409,091, 3,409,095, and 4,440,245 all to A. E. Bardwell--as well as in U.S. Pat. No. 2,872,158 to Green. In the drilling community, these multiple blow, gravity drop percussion drills are often referred to as chatter hammers due to their rebounding action, and these two designations will be used interchangeably throughout the present description.
In actual operation of the prior art devices, the weight, size, and severe operating conditions required of them combined to greatly limit the useful life of these drill types. As the devices were made stronger to withstand the rigorous operating conditions--often by fabricating them as sealed units--they became less amenable to routine repairs in the field. Substantial costs and weeks of delay were often encountered because when the springs contained inside a welded or completely sealed tube broke, there was no easy way for the driller to replace it in the field without the use of a machine shop. All steel components have, of course, a finite expected life and replacement of springs, cleaning out of normal wear and tear metallic debris, and other maintenance procedures must be provided for. This is precisely where the prior art devices fell short. For example, to gain access to these early chatter hammers required a lathe cutting away the welds used to join the various sections as well as the application of heat to swell the casing in order to remove the plugged end. This was not only very costly, but prevented quick reassembly due to the resulting changes in length of the tubing. These changes would necessitate the shortening of the hammer to assure the correct gap between the hammer and anvil--all of which is well beyond the capabilities associated with a field drilling team. Therefore, it is clear that a pressing need exists for an improved multi-blow percussion drill which yields the desired benefits of greatly increased rock penetration rates, and avoids the unduly short service life caused by poor capability of being maintained and adjusted under field conditions.